Previous work indicates that atypical lobules (AL) of the human breast resemble the precancerous hyperplastic alveolar nodules of mice, and statistical evidence indicates that AL may be precancerous also. Our purpose is to seek direct evidence which will prove or disprove the precancerous nature of AL and other human breast lesions. Individual lobules and lesions will be excised from fresh tissue and transplanted into the host gland-free mammary fat pads ("cleared fat pads") of athymic nude mice. The transplants will be observed for outgrowth and appearance and progression of anaplasia and/or neoplasia. In order to increase the possibility that outgrowth of human tissue will occur in nude mice, we will use various procedures, such as, treating the mice with anti-lymphocyte serum and hormones, treating the transplants with collagenase, and changing the topography of the cells by preculture followed by transplantation. If successful, the system will be useful for studies of the biological potential and physiological characteristics of normal and abnormal human mammary tissue.